Bitcoin is the king of cryptocurrencies.  Bitcoin fluctuates at its own pace.  Bitcoin does not depend on other cryptocurrencies to move its price.  There are many altcoins in cryptocurrency that we consider too risky to trust but we don't consider bitcoin too risky to trust because we have built trust with bitcoin.  We can easily rely on Bitcoin and invest as less risky.  And Bitcoin has a lot of future potential which is why Bitcoin investment is preferred by people.